The explanation involves a paradox. Erotica does so well because LibraryThing is a non-erotic site. The top subjects all win because of search-engine referrals. Google likes a mix of sites, so that erotic searches turn up something besides erotica. (This is particularly true if you have “safe search” enabled.) And LibraryThing has relatively high PageRank (PR), Google’s measure of a web site’s authority. Put these factors together and LibraryThing turns up high for erotic searches. For example, we’re currently Google’s number one site for “gay sex instruction.” Who would’ve thunk it?*

Of course, the “bounce rate” for these pages is astronomical. LibraryThing provides no actual sex instruction, just links to books about it—or rather links to metadata about books on sex instruction. That’s not what the searchers were looking for, and they leave as fast as they arrive.

As a side note, it’s sad to see so many top-level subjects in the list. I hope the bounce rate isn’t too high. Top-level subjects are where LCSH falls apart. Take a subject like “Historical fiction,” which has almost 8,000 works underneath it and no innate relevancy ranking. There can be little doubt–people don’t want to plough through 8,000 links!